Direct positive photographic materials



DIRECT POSITIVE PHOTOGRAPHIC MATERIALS Douglas James Fry and Bernard 'Alan Lea, Ilford, England, assignors to Ilford Limited, Ilford, England, a

British company No Drawing. Filed Feb. 23, 1962., Ser. No. 175,318 Claims priority, application Great Britain 10, 19 61 7 Claims. (Cl. 96-101) This invention relates to the production of direct positive photographic materials and more particularly to photographic emulsions adapted to be processed directly to positive images.

It is known that direct positive photographic emulsions may be made by including in a silver chloride emulsion a small proportion of a desensitising compound and fogging the emulsion by light or by chemical means. When such an emulsion is exposed and developed a positive image is directly produced.

Whilst the foregoing method of producing a direct positive photographic emulsion has received wide recognition and is extensively practised on a commercial scale, the actual production of such materials which measure up to the required standards is a difficult problem. In particular, it is found that the selection of the desensitising compound is critical and that not all desensitising compounds will give the effect to a useful degree, and very few indeed will afford products of commercial value. A great deal of research has been carried out therefore to find suitable desensitising compounds and many proposals have been made in the literature, of Varying value in practice.

Generally the emulsions have been coated on paper base and the products used for document reproduction. Among the desiderata for such emulsions for use for such purposes the following may be mentioned.

In the first place, it is desirable that the emulsion should have a good bleaching speed, i.e. the speed at which the fogged emulsion is reactive to the exposing light, so that in the exposed areas it affords (on development) a white background. However, since the materials are commonly used in offices and the like, without the use of a dark room, it is important that the effect of the general lighting on the exposed areas of the emulsion should not be such as to cause those areas again to become developable, i.e.

in the interval between imagewise exposure and development, since otherwise a grey or foggy background is oh- I ,tained instead of the clear white background which is preferred. This tendency to re-form a developable image in the exposure-bleached areas is referred to as the forward speed of the material, and in selecting a desensitising compound for use in direct positive photographic emulsionsof the type here in question, it is desirable to make the selection so that this forward speed is as small as practicable.

It also follows that since a print having a clear white background is desirable, the desensitising compound should not iself be so highly coloured that it imparts a discernible colour to the thinly coated emulsion layer.

Further, it is sometimes desirable to effect a double printing on a photographic material of the type here in question. Thus the material may be subjected to a first exposure by yellow light to a line image (black on a white background), thus bleaching the emulsion in the exposed areas. If the emulsion is developed at this stage United States Patent "ice 3,124,458 Patented Mar. 10, 1964 it will give a simple positive image on development, i.e. black lines on a white background. If instead of directly developing this exposed material it is re-exposed by white llght to a second original (again, for example, black lines on a white background), then in the areas which were bleached by the first exposure there will be formed, providing the forward speed of the material is 'sufiicient, a negative record of the second original superimposed on the first record. Thus the material when then developed Will show a record of the first original in black lines and a record of the second original in white lines. I

It is an object of the present invention to provide by the use of a specific class of desensitising compounds photographic direct positive emulsions which from the standpoint of the foregoing desiderata are especially sat isfactory.

Aceording to the present invention there is provided a direct positive photographic element which comprises a fogged gelatino silver halide emulsion of which .at least and preferably at least 99%, of the silver halide is silver chloride, which emulsion contains a desensitising compound which is a 2,2-pyridyl bis quaternary salt of the general formula:

carbon atoms, n is 2, 3 or 4, X is an anion, and either or both of the pyridine nuclei may carry alkyl substituent groups. The said emulsion may be fogged by light or by chemical means, e.g. treatment with formaldehyde or other reducing agent.

A preferred compound of the foregoing class for use in the present invention is 1,1'-ethy1ene-2,2'-dipyridylium dibromide (R ll, R=H, 11:2 and X=Br). Others are the analogous trimethylene (n=3) and tetramethylene (n=4) analogues and the 3',3-, 4,4- and 5,5'-dimethyl derivatives of the first-mentioned compound. The preparation of allthese compounds is. described by R. F. Homer and T. E. Tomlinson, Journal of the Chemical Society, 1960, page 2498. Other compounds of the class are the following (with their methods of manufacture):

1, l-( LI-DIMETHYLETHYLENE) -2,2'- DIPYRIDYLIUM DIBROMIDE 2,2-dipyridyl (0.8 g.) and 1,2-dibromo-2-methylpro pane (4 cc.) were heated together at C. for 15 hours.

The mixture was cooled, ether added and the solid filtered off and washed with dry acetone. It was dissolved in methanol, refluxed with charcoal, the solution filtered,

and the purified product obtained by adding ether to the methylpropane. The product had M. Pt. (decomp.) 290- The selected 2,2-dipyridyl bis quaternarysalt (which may be, for example, a bromide or other halide salt, or a sulphate, methane sulphonate or toluene-p-sulphonate salt) is preferably included in the photographic emulsion in an amount which is 0.1 to 2 g. of the compound per 100 g. of silver halide present. The emulsion may be prefogged by pre-exposure to light, but it is preferred to fog the emulsion chemically, for example by including formaldehyde in the emulsion and allowing the emulsion to stand at elevated temperature.

The following examples will serve to illustrate the invention:

Example I 4.4 litres of a washed photographic emulsion containing 188 g. of silver chloride and adjusted to pH 10 were treated with 12 ml. of a 4% aqueous solution of formaldehyde. The emulsion was heated at 52 C. for 35 minutes and was then neutralised by the addition of citric acid and cooled. There was then added 2 g. of 1,1-ethylene-2,2-dipyridylium dibromide dissolved in 100 ml. of ethanol. The emulsion was coated on paper base. On exposure and development the product yielded a direct positive image of high quality, of which the whites were unstained. The product after exposure and before development exhibited a relatively low forward speed.

Similar results were obtained with the other compounds of the aforesaid general formula set forth above, used in similar quantity.

Example 2 The procedure of Example 1 was followed but using only 0.1 g. of the said dipyridyl compound and the emulsion was coated on film base. The resulting film was exposed by yellow light to an original consisting of black lines on a clear background. 0n development it yielded a direct positive image of high quality.

A second portion of this film was exposed to the first original as before and then exposed by white light to a second original consisting of an arrangement of black lines on a clear background. The film was then developed and yielded a positive image of the first original and a superimposed negative image of the second original.

What we claim is:

1. A direct positive photographic element comprising a fogged gelatino silver chloride emulsion containing a desensitising compound selected from the class consisting of compounds of the formula:

where each R is selected from the class consisting of hydrogen and alkyl of 1 to 4 carbon atoms, n is selected from 2, 3 and 4, and X is an anion, and the said compounds in which at least one of the pyridine nuclei carries an alkyl substituent.

2. A direct positive photographic element according to claim 1 wherein the emulsion is chemically fogged.

3. A direct positive photographic element according to claim 1 wherein the emulsion is fogged by treatment with formaldehyde.

4. A direct positive photographic element according to claim 1 wherein the emulsion is fogged by light.

5. A direct positive photographic element according to claim 1 wherein the proportion of desensitising compound is O.1 to 2 g. per 100 g. of silver chloride.

6. A direct positive photographic element comprising a fogged silver chloride emulsion containing, as desensitising compound, a 1,1'-ethylene-2,2'-dipyridylium salt.

7. A direct positive photographic element comprising a fogged silver chloride emulsion containing, as desensitising compound, a 1,1-ethylene-2,2-dipyridylium salt in a proportion of 0.1 to 2 g. per 100 g. of silver chloride present.

References Cited in the file of this patent UNITED STATES PATENTS Dufiin et al Dec. 20, 1960 Fry et a1. May 22, 1962 OTHER REFERENCES 

1. A DIRECT POSITIVE PHOTOGRAPHIC ELEMENT COMPRISING A FOGGED GELATINO SILVER CHLORIDE EMULSION CONTAININGD A DESENSITISING COMPOUND SELECTED FROM THE CLASS CONSISTING OF COMPOUNDS OF THE FORMULA: 